For lung cancer assessment, the physician needs to perform a biopsy of the suspect cancer sites, such as the peripheral nodules or mediastinal lymph nodes. Such sites are first identified by analyzing the 3D CT image data of the chest. Later, during bronchoscopy, the physician attempts to reach these sites with the help of the live video obtained from a bronchoscope. The success of a standard bronchoscopy depends heavily on the skill level and experience of the physician. The success of the bronchoscopy could be increased if the physician received some form of guidance during the procedure.
Several guidance methods have been suggested in the past few years [1-5]. All of them use a CT-based (virtual) endoluminal rendering of the airway surface to obtain both the depth and visual data. They try to find the 3D location and orientation of the bronchoscope (pose) using the virtual renderings and incoming video frames. Bricault et al. proposed a method to register the bronchoscopic video (real) and 3D CT virtual bronchoscopic images [1]. The method uses the segmentation and shape from shading techniques to find the 3D surface for the real image and then does a 3D-3D registration of the computed surface with the virtual surface.
Mori et al. proposed a method which first tracks a set of points across the real frames to estimate the bronchoscopic motion by computing the essential matrix and then does an estimation of the residual motion using image registration by Powell's method [3]. In [5], Mori et al. use a Kalman filter to predict bronchoscope motion and a new similarity measure to reduce the image area to be registered. Helferty et al. use a coarse tracking and fine registration approach [2,6]. The tracking is implemented by using the standard optical-flow constraint equation and depth-map information from the virtual rendering to estimate the motion parameters. The registration is done by maximizing the mutual information between the real and virtual image using the simplex method.
The method proposed by Bricault et al. does not involve tracking and is limited to the bifurcation images [1]. The method of Mori et al. computes the essential matrix for tracking [3] and Powell's method for registration. The approach has three limitations. Firstly, the use of Powell's method makes the registration step slow. Secondly, the essential matrix cannot be determined if a subset of points are coplanar [7]. Thirdly, a translation can only be recovered up to a scale from the estimated essential matrix [7]. The optical-flow approach taken by Helferty et al. for tracking is slow since it involves iterative warping and computation of gradients for the images [2, 6]. Use of simplex method makes the registration step slow as well.